silvics and site limiting factors

silvics

the study of life, history, and general characteristics of forest trees and stands

give us insight into where a species will grow, how it will respond to enviro factors / management practices, and limiting factors

why are knowing site factors important?

for making good silviculture decisions - such as regarding species, site prep, silv system, tending, age to tend, etc

consider biotic and abiotic factors, conditions vary year to year, place to place, day to day, etc

indirect site factors

insects, disease, wind, physical interactions, chemical interactions (allelopathy), browsing / wildlife, vegetation competition

direct site factors

light, water, nutrients, temperature

light - photosynthetically active radiation

wavelengths of 400 - 700 nm

usually expressed as % light transmissions in forests

ex min light for survival: Sw 8-15%, Pl 20%, Fb 8%

ex for Sw as we increase sunlight transmission growth increases and seedling mortality decreasessh

shade tolerance

ability of a tree to germinate, establish and survive in the shade

• ability to survive low light, not needing shade

related to photosynthesis - more tolerant species have lower compensation points, lower saturation points and can adapt crown morphology to optimize light capture under low light conditions

improves stand management by ability to characterize trees, understand species-specific requirements and location requirements

examples of shade intolerant species

jack pine, tamarack, trembling aspen, balsam poplar

examples of shade tolerant species

balsam fir - more than the others

white spruce, black spruce

factors influencing understory light

time of year, location, weather, overstory species, foliage amount, crown closure

Beer’s Law - overtopping veg cover

drought and dry sites

water stress reduces growth, severe stress causes mortality

reduces stomatal opening, net CO2 uptake and photosynthesis

trees allocate more carbon below ground to find more water, for silviculture we want the above ground growth

• stand leaf area declines and long term stem-volume decreases - more energy to fight stress than grow

• increases allocation to roots and less to stems

species with high drought tolerance

ponderosa pine, jack pine, lodgepole pine, douglas fir

species with moderate drought tolerance

aspen, western larch, white spruce, western white pine, black spruce

species with low drought tolerance

western red-cedar, western hemlock, balsam fir, balsam poplar

would reducing competition increase water available to crop trees?

understory veg may reduce water from crop trees, but also provide important ground cover - minimize but not eliminate

lots of different factors at play, most of the time we would want to reduce the competition

how does excess water impact growth?

primary impact is soil aeration, O2 is depleted after flooding

different species more or less tolerant to flooding - more = Sb, tam, willow, Pb, or less = Sw

impact on species selection, microsite, site prep - what trees survive where

flooding varies across sites, season, year, climate, water table increases with harvesting - and things change over time, adaptive management

nutrients

macros include N, P, K, Ca, Mg, S - required in larger quantities

micros include Fe, Cu, Zn, B, Mo, Cl

limitations can be assessed by foliar nutrient concentrations

factors influencing nutrient availability

parent material - mineralogy and texture

soil temperature - decomp and mineralization rates

soil moisture - decomp rate, ability to uptake nutrients

disturbance history - assart effect, fires may be consuming SOM and aspen can promote rapid nutrient cycling

other vegetation - N fixers

assart effect

release of accumulated nutrients occurs after disturbance (reduced uptake and increased mineralization)

management to change nutrient availability

fertilization

veg management - brushing, herbicides, spacing

manipulation of SOM - burning, removal, mixing to reduce availability

intensive levels of utilization - short rotations, harvesting or removal of branches and foliage, longer term impact on certain sites

temperature

major climatic factor influencing distribution and growth of trees - also includes for soil and air, = the function of climate, season, weather, elevation, aspect, veg, height, soil properties

influences growth rates → effects on photosynthesis, respiration, growing season length, etc

extreme can damage tissues

extreme heat

air temps >50C can damage or kill tissues

more extreme in clearcuts, no shade from other veg to reduce

management options - shelterwoods, obstacle planting or shade cards

extreme cold

frost during active growing season can kill fresh tissues

conifers susceptible following bud burst

high frost tolerance: Pj, Pl, Sb, moderate tolerance: Sw, Se, low: Fd

use knowledge to inform which species you plant, what you prescribe to an area - ensure survival, not all species respond in the same way

radiative frost

occurs on clear nights when heat loss > inputs

overtopping veg reduces frost exposure

advective frost

occurs in depressional areas (pockets) due to the pooling of cool air (dense cold air settles in low areas)

so in depression areas we wouldn’t want to plant frost intolerant trees - microsite scale

winter injury

when roots are frozen but shoots are active

occurs in early spring when warm sunny periods occur but soil is still frozen, desiccation occurs and impacts root activities, difficult for the plant to absorb water

ex chinook conditions imp to consider

need to consider planting timing, shelter, site prep, natural regeneration, provenance (where the seeds are from)

soil temperature

often a limiting factor - limits root growth and water uptake, restricts flow to roots, delay buds, reduced mineralization by microbes

site prep and competition control (herbicide and thinning) can increase soil temp

frost heaving

frost heaving

results when ice lenses form in the soil and push soil and seedlings upwards

most susceptible is fine textured soils with high soil water content and no surface OM

root damage can reduce growth and survival of conifers

reduced by: retaining veg cover, retaining mulch / surface OM, deep planting of large stock

vegetation competition impact

light - taller plants reduce light for smaller

water - species water utilization and rooting characteristics

nutrients - macros most essential for plants

space - particularly belowground

snow press, etc

higher density =/ competition - facilitation and other beneficial impacts can also occur

• ex soil conditions can be improved, nutrients added to soil, better light conditions, etc

identifying limiting factors

law of the minimum - what specifically is missing and can be improved

observation - trees, veg, soils, topographic position

experience with similar sites and conditions

and then use your tools

what makes good planting stock?

consider above and belowground factors, seed lot factors

tall is not always better, judge the whole seedling

roots are really important right after planting - must restore water uptake to support functioning

proper handling is important, root growth capacity - ability of a seedling to produce new roots under test conditions - doesn’t always inform field survival